The field of the present invention is that of rotating electrical machines, especially dynamo-electrical machines such as automotive alternators (generators).
Rotating electrical machines such as automotive alternators (also commonly referred to as xe2x80x9cgeneratorsxe2x80x9d) have two spaced apart frames which provide the main structural elements of the alternator. The frame closest to a pulley, which powers the alternator via a belt drive, is commonly referred to as the drive end frame. The opposite frame is commonly referred to as the slip ring end frame. The two frames support between them a rotor shaft with a connected rotor winding. The frames are held together typically by three or four bolts which are attached axially between ears or bosses on the outside of the frame.
Each frame has a hub. The hub includes an inner core having a central axial opening (sometimes referred to as the bearing bore). The inner core axial opening provides mounting support for an outer race of a roller bearing which mounts the rotor shaft to the hub. The outer race of the bearing is typically press fitted within this central opening of the core. Extending radially outward from the core are a series of hub ribs forming between themselves ventilation openings. The hub ribs connect the core with a rim of the hub.
Mounted on the shaft of the rotor in a position adjacent to the hub will be a fan. During operation of the alternator, fan blades pass close by the hub ribs, essentially shearing the air as the blades pass near the hub ribs. The result is xe2x80x9cairxe2x80x9d noise from the fan and hub rib interaction. Spacing or stagger of the hub ribs and fan blades is important since resonance and harmonics occur due to the fan and hub rib interaction resulting in an amplified noise level. Also, the distance between the fan blades and the hub ribs is critical. The closer these features are positioned to each other, the higher the air flow and the greater the interaction which results in an increased air noise.
A second effect of the hub ribs is on bearing vibration resulting in mechanical noise. The bearing itself is a source of vibration, due to the balls moving between the races in a dynamic setting. As the bore that the bearing race is press fitted into takes an out-of-round, the more mechanical noise typically occurs. The bearing bore (central axial opening) is machined round, with a tolerance that is fairly tight.
However, as mentioned previously the bearing is assembled within the bore as a press fit. During the application of interference, any hub geometry that is not cyclically symmetrical will result in the bearing and the bore taking a shape that is more deformed than initially as machined. The hub ribs are critical to this as they attach to the bore at discrete locations, basically providing a series of stiffer sections around the hub of the frame. The effect of the hub ribs is to cause the bore to become less round as a function of the stiffer sections.
It is desirable to provide a rotating electrical machine with hub ribs wherein air noise and mechanical noise are diminished, while at the same time maximizing air flow through the opening between the core and rim.
The present invention brings forth an electrical machine having a frame with a hub with an improved rib design which lowers air and mechanical noise. The hub ribs are angled from the axial direction to improve fan performance (air flow and cooling) as air is redirected from the axial direction through the hub ribs to a rotating radial direction through the fan blades. Tilting the hub cross-section from the axial also spreads the radial stiffness induced by the rib over the hub about a larger angle in the rotational direction. Therefore, roundness of the press fit bearing race is improved. With roundness being improved, vibration is reduced and mechanical noise is lessened.
Other features of the present invention will become more apparent to those skilled in the art after a review of the drawings and the accompanying detailed description.